regexp: leave a pointer to resurrect workaround for Homebrew

regexp: leave a pointer to resurrect workaround for Homebrew

Recently some GitHub CI jobs were broken by update on the platform
side, which was eventually resolved by image rollback, but in the
meantime Dscho invented a workaround patch to sidestep the broken
part of the platform.  Their future image update may contain the
same bug, in which case the workaround may again become needed.

As we do not want to be building with workaround that avoids system
regexp library altogether unless the system is known to be broken,
so short of an automated "detect broken system and apply workaround"
mechanism, let's use the folks who are compiling the code to detect
breakage on their system and cope with the breakage ;-)

Signed-off-by: Junio C Hamano <gitster@pobox.com>